An obstacle is moving towards the source with velocity v. The sound is reflected from the obstacle. If c is the speed of sound and λ is the wavelength, then the wavelength of the reflected wave λr is (a) λr = ((c-v)/(c+v))λ (b) λr = ((c+v)/(c-v))λ (c) λr = (c-v/c)λ (d) λr = (c+v/c)λ

Answer: (a). Obstacle as moving observer: f₁ = f (c + v)/(c). Obstacle as moving source towards the original source: fr = f₁ (c)/(c - v) = f (c + v)/(c - v). λr = c/fr = ((c - v)/(c + v))λ So the answer is (a).

An obstacle is moving towards the source with velocity v. The sound… | MHT CET 2020 (16 Oct, Shift 2)

Q47MHT CET 2020 · 16 OCT · SHIFT 2Medium

Doppler Effect

An obstacle is moving towards the source with velocity $v$ . The sound is reflected from the obstacle. If $c$ is the speed of sound and $\lambda$ is the wavelength, then the wavelength of the reflected wave $\lambda_r$ is

Show answer and solution

Answer: (a)

Obstacle as moving observer: $f_1 = f\,\dfrac{c + v}{c}$ .
Obstacle as moving source towards the original source: $f_r = f_1\,\dfrac{c}{c - v} = f\,\dfrac{c + v}{c - v}$ .
$\lambda_r = \dfrac{c}{f_r} = \left(\dfrac{c - v}{c + v}\right)\lambda$
So the answer is (a).

Report a mistake

Doppler Effect question from MHT CET 2020 (16 Oct, Shift 2)

More questions on Doppler Effect